DETROIT (Reuters) - General Motors Co said on Wednesday its Chevy Volt plug-in hybrid will carry an overall fuel economy rating of 60 miles per gallon, topping the Toyota Prius, the industry’s long-time fuel economy leader.

GM has begun to assemble the long-awaited Volt at a Detroit-area factory, with initial sales expected in December.

The EPA fuel-economy rating, which will be displayed on stickers on each Volt sold, was one of the last hurdles for a vehicle that GM has made the center of its effort to restart its reputation for technology and innovation.

The Volt has been named Green Car of the Year and Motor Trend car of the year in recognition of the automaker’s four-year effort to develop a first-of-its-kind, mass-market hybrid that can run on both electric power and gasoline.

The release of the U.S. Environmental Protection Agency rating on the Volt came as GM rolled out a television commercial to thank Americans for its taxpayer bailout, a week after an initial public offering of GM shares.

The tagline of the ad reads: “We all fall down. Thank you for helping us get back up.”

Capturing the fuel-economy leadership from Toyota Motor Corp’s hybrid Prius will give GM bragging rights it has sought throughout the Volt development effort.

But the Volt’s EPA fuel economy ratings released on Wednesday also highlight aspects of the vehicle’s performance that GM had been accused of overhyping.

In August 2009, just after it emerged from bankruptcy, GM ran a series of ads to trumpet that the Volt was capable of getting 230 miles per gallon in city driving based on a preliminary EPA test.

The EPA gave the Volt a “miles-per-gallon-equivalent” -- or MPGe -- rating of 93 in all-electric mode, just below the rating of 99 for the Nissan Leaf assigned earlier this week.

The EPA also rated the Volt’s range when powered by the 400-pound lithium-ion battery pack at 35 miles, less than the 40 mile range GM had used in its earlier descriptions of the car.

Nissan Motor Co Ltd’s battery-powered Leaf, which also goes on sale next month, has an EPA-certified battery-only range of 73 miles.

GM said it would continue to describe the Volt’s battery range as between 25 miles and 50 miles, depending on conditions.

For longer trips, and in some other situations, the Volt also has a 1.4-liter engine that kicks in to give drivers an estimated 379 miles of total driving range, said Doug Parks, the GM executive in charge of electric car projects.

BRAGGING RIGHTS AT STAKE

Running on gasoline alone, the Volt will have a traditional EPA fuel economy rating of 37 miles per gallon in combined city and highway driving.

After accounting for the share of the average trip expected to include some pure electric driving, the EPA gave a combined 60 miles-per-gallon rating to the Volt, GM said.

By comparison, Toyota’s 2011 model-year Prius has an EPA rating of 51 miles per gallon in city driving and 48 in highway driving.

In creating fuel economy rankings for upcoming vehicles like the Nissan Leaf and the Chevy Volt, the EPA and automakers struggled to find labels that would convey more information than traditional stickers without overwhelming consumers.

The Volt label, for instance, gives its projected cost per year in all-electric mode ($601) and in all-gas mode ($1,302).

As a pure-electric car, the Leaf tops the Volt in the category on the EPA label that tracks greenhouse gas emissions from the vehicle.

Since it carries no combustion engine, the Leaf has no such emissions although greenhouse gases would be produced by the power plants used to recharge the car.

The Volt is rated at 84 grams of carbon dioxide per mile, less than one-tenth of the industry’s worst-performing vehicle on that score.

GM had said it would build 10,000 Volts in 2011 but has said more recently that output would be increased to meet strong demand.

Governments in Europe, Asia and the United States are offering subsidies to consumers and producers of a wave of electric vehicles, soon to include offerings from Ford Motor Co and Toyota.

Sales of electric cars, including plug-in hybrids like the Volt, are expected to account for a single digit share of auto sales over the next decade, according to most industry estimates.

Reporting by David Bailey and Kevin Krolicki; Editing by Steve Orlofsky and Tim Dobbyn